A BlockBasedSource is a FileBasedSource where a file consists of blocks of
records.

BlockBasedSource should be derived from when a file format does not support efficient
seeking to a record in the file, but can support efficient seeking to a block. Alternatively,
records in the file cannot be offset-addressed, but blocks can (it is not possible to say
that record {code i} starts at offset m, but it is possible to say that block j
starts at offset n).

The records that will be read from a BlockBasedSource that corresponds to a subrange
of a file [startOffset, endOffset) are those records such that the record is contained in
a block that starts at offset i, where i >= startOffset and
i < endOffset. In other words, a record will be read from the source if its first byte is
contained in a block that begins within the range described by the source.

This entails that it is possible to determine the start offsets of all blocks in a file.

Progress reporting for reading from a BlockBasedSource is inaccurate. A BlockBasedSource.BlockBasedReader reports its current offset as (offset of current block) + (current block
size) * (fraction of block consumed). However, only the offset of the current block is required
to be accurately reported by subclass implementations. As such, in the worst case, the current
offset is only updated at block boundaries.

BlockBasedSource supports dynamic splitting. However, because records in a BlockBasedSource are not required to have offsets and progress reporting is inaccurate, BlockBasedReader only supports splitting at block boundaries.
In other words, BlockBasedSource.BlockBasedReader.atSplitPoint returns true iff the current record is the
first record in a block. See FileBasedSource.FileBasedReader for discussion about split
points.